1. Field of the Invention
The present invention relates to a printing apparatus which prints on a printing medium by transferring droplets of an ink held in an ink storage container such as an ink cartridge.
2. Description of the Prior Art
Various types of printers which print with a liquid ink in place of ink ribbons and the like have recently been proposed. In the field of output devices for computer systems, such ink-jet printers are becoming prevalent.
In an ink-jet printer, the volume of a nozzle is changed by a piezoelectric element so as to inject fine droplets of ink through an orifice at the distal end of the nozzle, thereby printing recording dots on a printing medium.
An ink-jet printer produces less noise than an impact printer. However, since the ink is in liquid form, handling of the ink is difficult in an ink-jet printer as compared to a printer which prints with an ink ribbon. Especially, some special mechanisms must be incorporated including a mechanism for preventing formation of bubbles or solids in an ink feed channel or recovery upon formation of such bubbles or solid, or a mechanism for guaranteeing reliable ink feed to a printing head.
FIG. 1 shows the structure of a printing head portion of a conventional ink-jet printer.
Referring to FIG. 1, a carriage 1 is reciprocated by a drive means (not shown) along a guide shaft 2 for printing. A printing unit 7 is mounted on the carriage 1. An ink-jet head 4 comprising a piezoelectric element as described above is arranged on a surface of the carriage 1 which faces a printing medium. Upon being driven, the ink-jet head 4 injects an ink in a tank 3 which a part of the printing unit 7 toward the printing medium.
The tank 3 is divided by a thin tube portion 3C into an ink storage portion having a vent hole 8 at the right, and an ink feed portion for the ink-jet head 4 at the left. The tank 3 has this construction so as to minimize the ink level fluctuation upon reciprocal movement of the carriage or formation of bubbles.
In a conventional printer, in order to feed ink to the ink-jet head 4 reliably, electrodes 5A and 5B are formed in the ink feed portion of the tank 3 for feeding the ink to the ink-jet head 4 and in the ink storage portion. Conduction between the electrodes 5A and 5B through the ink is detected by a comparator or the like to detect the ink level in the tank 3. Then, a warning LED 15 or the like mounted on the head or the like is turned on to signal to the operator the need for ink replenishment.
In the printer shown in FIG. 1, the electrode 5A at the left is arranged immediately above the ink-jet head 4. When an ink level 3A reaches an ink detection level 3B corresponding to the position of the electrode 5A, the warning LED 15 is turned on so as to signal to the operator a decrease in the ink level.
Then, the operator opens an opening 9 of a replenishment cartridge 6 to replenish fresh ink into the tank 3 and to recover the ink level in the tank 3 to the normal level. However, bubbles may have formed inside the ink-jet head 4 due to a previous drop in the ink level. Therefore, the bubbles cannot be removed by only the pressure acting upon level increase in the tank 3. For this reason, the printing quality may be degraded for a short period of time after ink replenishment.
In addition to the decrease in the ink level upon frequent use of the printer, the ink level may also be decreased due to evaporation of water in the ink when the printer is not used for a long period of time. In this case, since the ink viscosity has increased, it is particularly difficult to remove bubbles by an increase in the ink level. It also takes a considerable period of time for the ink to recover the original viscosity.
In order to prevent formation of bubbles and an increase in the ink viscosity, it is preferable to keep the ink level 3A in the tank 3 as high as possible. However, when the ink detection level 3B is raised for this purpose in the tank of the conventional construction, the ink replenishment must be performed too frequently.